Obstructive sleep apnea (OSA) is a disorder in which the respiratory tract is physically obstructed during sleep, and respiration stops for as long as two minutes. Blood pressure temporarily rises in an apnea attack (referred to hereinafter as a nocturnal blood pressure surge). For example, a blood pressure that is normally 120 to 130 mmHg rises to over 200 mmHg. Accordingly, each time an apnea attack occurs, there is the risk of sudden death, stroke, or a cardiovascular event such as heart failure. The ability to accurately measure a nocturnal blood pressure surge is therefore in demand.
When measuring a nocturnal blood pressure surge, the blood oxygen saturation level decreases when respiration stops. In view of this, Patent Literature 1 (JP S62-155829A) and Non-Patent Literature 1 (A New Technique for Detecting Sleep Apnea-Related “Midnight” Surge of Blood Pressure, Shirasaki et al., Hypertens Res Vol. 29, No. 9 (2006), p. 695-702) propose methods in which the blood oxygen saturation level is measured with an oximeter, and blood pressure measurement is started when the measured blood oxygen saturation level falls below a set reference.